You only have one mic so mono should be fine!
Ask yourself what would be the point of recording exactly the same on both channels.
In post you should interpret the audio source as mono.
The Summilux 15mm or the 12mm would be a good pick. Although the latter is rather pricey.
GH5 sensor will not be bigger but they will use the whole sensor area for 4K.
Okay, that's a wrap, I take the Arri Zeiss Master Prime set!
Please let me know if there is a coupon with 99% discount! :)
I would recommend you use prime lenses only. The above mentioned lens is relatively heavy and a problem to balance.
Your prospect is not good. Going from 30p -> 24p is nasty whatever method you wind up using. Reverse telecine only works if it was originally recorded in 24p and telecined to 29.97.
24p to 25p is not a problem.